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Functions, Procedures, and
Abstraction
Dr. José M. Reyes Álamo
Status of Assignment and Quizzes
Past Due
• RA1_ComputerConcepts
Pending
• RA4_PythonCh3Ch6
• RA2_PythonCh1Ch2
• RA5_PythonCh8Ch10
• RA3_PythonCh5Ch7
• Labs
• Quiz1
– Lab1
– Lab2
– Lab3
– Lab4
2
Outline
• Functions
• Built-in functions
• User defined functions
• Abstraction
• Reusability
• Parameters and arguments
• Returning values
• Variables Scope
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Functions
Functions
• From mathematics, a functions perform
some operation and returns a result.
• Functions hide the details of and operation,
to make it easy to use by others
–e.g. sqrt() for computing the square root.
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Function
• In the context of programming, a function is a
named sequence of statements that performs a
computation.
• When you define a function, you specify the name
and the sequence of statements.
• You can invoke the function by name.
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Built in functions
Python provides built-in functions
• Type conversion functions
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A big list of built-in functions
http://docs.python.org/library/functions.html
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Remember the Math module?
• Math functions
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Lots of Python modules available
• Built-in modules
– The Python Standard Library:
http://docs.python.org/library/
• Other modules
– PyPI - the Python Package Index
http://pypi.python.org/pypi
– The Python Package Index is a repository of software
for the Python programming language.
– There are currently 24018 packages there.
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User defined functions
Mathematical notation
• Consider a function which converts
temperatures from Celsius to temperatures in
Fahrenheit:
– Formula: F = 1.8 *C + 32.0
– Functional notation: F = celsisus2Fahrenheit(C)
where
celsius2Fahrenheit(C) = 1.8*C + 32.0
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Function definition
• Math:
–f(C) = 1.8*C + 32.0
• Python:
• Terminology: parameter “C”
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© 2011 Pearson Addison-Wesley. All rights reserved.
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Definition and calling
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Triple quoted string in function
• A triple quoted string just after the def is called a
docstring
• docstring is documentation of the function’s
purpose, to be used by other tools to tell the user
what the function is used for.
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Abstraction
Abstraction
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Abstraction
1. The ability of dealing with ideas rather than
events.
2. Something that exists only as an idea.
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Abstraction in computing
• A programmer would use abstraction to define
general purpose functions.
• Abstraction is one of the most important
techniques in software engineering as it is
used to reduce complexity.
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Reusability
Why have functions?
• Support divide-and-conquer strategy
• Abstraction of an operation
• Reuse: once written, use again
• Sharing: if tested, others can use
• Security: if well tested, then secure for
reuse
• Simplify code: more readable
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How to write a function
• Does one thing. If it does too many things, it
should be broken down into multiple functions
(refactored).
• Readable. You should be able to read it as well
as others.
• Reusable. If it performs its task well, you can
reuse.
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More on functions
• Complete. A function should check for all the
cases where it might be invoked. Check for
potential errors.
• Not too long. As it does one thing, code is usually
succinct.
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Parameters and arguments
Parameters vs argument
• A parameter is the variable which is part of the function's
definition.
– Inside the function, the arguments are assigned to
the parameters.
• An argument is an expression, value, or literal used when
calling the method.
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celsius=25
val=25* 1.8 + 32 = 77.0
© 2011 Pearson Addison-Wesley. All rights reserved.
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Returning values
Return Statement
• The return statement indicates the value
that is returned by the function.
• The return statement is optional. If there is
no return statement, the function is often
called a procedure.
• Procedures are often used to perform
duties such as printing output or storing a
file
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Multiple returns in a function
• A function can have multiple return
statements, but only one will execute.
• The first return statement found,
executes and ends the function.
• Multiple return statements can make
your code confusing, therefore should
be used carefully.
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Multiple return statements example
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Variables scope
Local variables
• When you create a variable inside a function, it is local,
which means that it only exists inside the function. For
example:
• This function takes two arguments, concatenates them,
and prints the result twice. Here is an example that uses it:
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Local values
• When cat_twice terminates, the variable cat is
destroyed. If we try to print it, we get an exception:
• Parameters are also local. For example,
outside cat_twice, there is no such thing as
part1 or part2.
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Summary
• Function:
– Named sequence of statements that performs a computation
• Built in functions:
– Python provides built-in functions and modules. Lots of extra
modules available
• User defined functions:
– It is also possible to add new functions. A function
definition specifies the name of a new function and the
sequence of statements that execute when the function is
called.
• Abstraction:
– The ability of dealing with ideas rather than objects or events.
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Summary
• Reusability:
– Functions can be re-used in another program.
• Parameters and arguments:
– A parameter is the variable which is part of the function’s
definition. An argument is the value or expression used when
calling the method.
• Returning values:
– The return statement indicates the value returned by the
function. A function with no return statements is often called a
procedure.
• Variables Scope:
– Parameters and variables created inside a function are local,
which means that they only exists inside the function
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Status of Assignment and Quizzes
Past Due
• RA1_ComputerConcepts
Pending
• RA4_PythonCh3Ch6
• RA2_PythonCh1Ch2
• RA5_PythonCh8Ch10
• RA3_PythonCh5Ch7
• Labs
• Quiz1
– Lab1
– Lab2
– Lab3
– Lab4
38